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Did Taiwan Shake All Day?

Matt Alsdorf
Posted Friday, Sept. 24, 1999, at 7:08 PM ET

The day after Taiwan's massive earthquake, newspapers reported that there had already been more than 2,000 aftershocks. Even using conservative estimates, this implies at least one aftershock every minute for more than 24 hours. Could this be true?

Yes. Most large earthquakes produce thousands of aftershocks, but the majority of them are imperceptible. And although Taiwan's reported rate is above average, it is not considered extraordinary.

Earthquakes are caused by the sudden release of pressure that has built up between two tectonic plates. Because the stress is rarely discharged in a single jolt, earthquakes almost always occur in clusters: first, a few small foreshocks; then the strongest quake, or mainshock; then aftershocks. An aftershock is defined as a quake that occurs on or near the original fault (usually within 10 miles of the epicenter) and before the region's seismic activity has returned to normal.

Some faults, such as the San Andreas in Southern California, are seismically active long after a major quake and can experience aftershocks up to a decade later. But most aftershocks happen soon after the primary quake. In fact, seismologists have a rough method for predicting the number of aftershocks on a given day: It declines in inverse proportion to the time since the mainshock (i.e., day four has one-fourth the number of quakes as day one).

Experts estimate that 90 percent of aftershocks are below 3 on the Richter scale, and not generally perceived by people. (Click here for a good explanation of the Richter scale and here for the effects of various quakes.) In the three hours following Taiwan's mainshock, for example, there were only seven quakes that would have done more than lightly rattle dishes. So, Taiwan was technically shaking all day. But most people probably didn't know it.

Next question?

Explainer thanks Slate reader Tom Castle for submitting this question, and Professor Leon Peng of the Southern California Earthquake Center and Bill Smith of the U.S. Geological Survey Earthquake Information Center for helping to answer it.

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Matt Alsdorf is a Slate editorial assistant.
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